Add open-policy-agent gatekeeper as an optional Addon

[scheeles]

What this PR does / why we need it:
Enables the possibility to use OPA policies via the gatekeeper on user-cluster

Documentation:
kubermatic/docs#323

Add open-policy-agent gatekeeper as an optional Addon

[kdomanski]

/retest

[kdomanski]

/lgtm
/approve
/retest

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 3379963c664de8b6aa72fce9eae6b2e1634c37d5

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kdomanski, scheeles

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[kdomanski]

/test pre-kubermatic-e2e-aws-flatcar-1.18

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[scheeles]

/test pre-kubermatic-e2e-operator

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